AMi Continental Hum

Hi,

I have an AMi Continental (mono) jukebox that has developed a loud hum.

It is not too loud at the beginning of a song, but, at the record end (between the record end and the mute switch turning off the amp), it has a very loud hum.

I know this is an impossible question, but, do 'hums' generally mean it requires new capacitors or tubes (or both)?

Any advice appreciated.

Regards

Gordo..

Considering the age of the jukebox, if the amplifier has NOT been rebuilt, then I would certainly start by recapping it now. In particular with the electrolytic capacitors first then onto the wax/paper styles. Also it would be a good idea to measure all the resistors to look for any that have drifted out of tolerance or gone way off value.

Tubes could play a part in the hum problem (rectifier especially) so they ought to be tested. However, I think you're more likely to find some bad capacitors.

Those machines have a very wide AGC compression range -on the order 25 db. Any resistance that has built up in the pickup shield circuit or on the connections to the cart can cause a reduction of output from the cart and allow stray fields to induce a hum. This will always be 60Hz type. The AGC on these machines is very slow to increase gain (long time constant) therefore it can take upwards of one minute for the gain to build up to maximum.

Disconnect the pickup RCA plug from the amp input, set the machine on play and see if the hum has gone.

If not the problem is obviously internal to the amp.

The type of hum can clue you in as to what is going on.

60Hz hum will be grounding or heater to cathode leakage.

120Hz hum will be filter related. Either the filtering caps have lost capacitance or there is excessive drain on the power supply that is reducing the efficacy of the filters.

Either way, as Ken points out, that amp is way overdo for a rebuild.

Rob/NYC

Thanks for your replies.

I should add that when I bought the juke (a few years ago) the seller told me that the amp had been rebuilt (I don't know if this is correct or not). However, there are no 'old' wax caps in this amp. It sounds great (except for the hum). Plenty of volume and very clear. I installed a Stanton 400 V3 cartridge about 18 months ago. All the tubes are lit and shine brightly.

Rob as you advised, I removed the pickup plug from the amp. No sound at all, the hum is not there with the plug removed. What could this indicate? Does it tell me anything?

As I am not a technician, please keep any advice (very) simple. I live in Australia and have no access to any jukebox technicians so it must be 'teach myself'.

Thanks again for your time.

Kind Regards

Gordon..

Gordo, that tells you that the problem is within the pickup wiring.

One thing to check is the tiny copper strap that goes from one of the pins on the cart to the shell of the cart. It must be removed when the cart is installed into a metal arm as it produces a ground loop.

Also check for any instances of the braided shield on the pickup wire is touching the arm shell or any part of the mech. That wire must "float" and only be grounded at the amp. On the two Continentals and one "I" -I removed the old tonearm wires and used new unshielded wire and came out to an RCA plug at the arm end of the run to the amp.

Sometimes the pin clips lose good contact with the pins on the cart.

Anyway, your problem is simple wiring.

Rob

Gordo,
Real simple thing to try first--
Remove the RCA plug from the amp--Gently, squeeze the outside contacts on that plug, just a little tighter--try it now--this type plug is (in)famous for loosing contact right there--
Ron Rich

Hi Ron, Rob and Ken,

Thanks for taking the time to reply.

I fixed the problem and now it has no hum at all. THANKS!!

I followed all your advice and now it is working great and sounds fantastic. I think it was the RCA plug, although the metal strap on the cartridge was still in place (I have now removed it).

Another thing I noticed while fixing this problem....The juke is on free play and when I initially turn the power on, the latch bar solenoid activates and the 12AU7/ECC82 tube glows very bright orange, as if too much voltage is going through it. It settles back to 'normal' after a second or so.

Could this be the start of other problems?

Thanks again, I would be lost without your help.

Gordon....

Gordon. the transient flash on the tube is due to uneven resistance in the heater. I have several 12AX7's in location machines and the amps are set up so that the tubes go totally off at the end of each session. This on-off flashing occurs way more times than you'll have at home and there are no problems associated with it.

IIRC: On the Continental the latch solenoid pulls-in at power up to release the latchbar. Odd, but that is how AMI designed it. There shouldn't be a problem as long as the microswitch that inserts a series resistor is set up correctly.

Glad you are now "hum free".

Rob

Rob, Gordo,
AMi designed that latch bar to energize, for an instant, on power up, because it was possible to lock the keys down when power was off. Should that happen, and a "credit" had been left unused, the phono would automatically play the selection of the "locked keys" the instant the power was turned on. This "system" worked the opposite of all other systems of that day--this was done to stop latch bar coils from "catching fire" as they needed to fire only instantly, at the end of each selection, rather then remain "hot", whenever credits were established. Actually, IMHO, it was a great system-- Ron Rich

Ron, I agree that the design had potential, however AMI also setup the power distribution such that throwing the power switch off only turned off the lights and left the electronics partially energized.

I was told the reason for this was to prevent some idiot from losing his money if he/it was dumb enough to put money in a dark machine.

As you might expect, when I rebuilt my two Continentals that was changed.

Rob

Hi Rob,
Yes--I never understood why the "main power switch", should be so wired--I understand part of it was to keep the amp hot, so a mic could be used if the juke was on, or off---I'm not sure, but my memory says that the "Joe Hole" could deposit his money and make selections with the switch off--but the phono would not play them till the switch was turned on ??
Ron Rich